Cryogenic refrigerators, such as those incorporated in cryogenic vacuum pumps (cryopumps), commonly are of a "Gifford-McMahon" design. Under standard operation, a two-stage cryogenic refrigerator of this design can typically cool to extremely low temperatures--typically, 4 to 25K.
A refrigerator that performs a Gifford-McMahon cooling cycle is illustrated in FIG. 1. The refrigerator includes a displacer 12 including a first stage 14 and a second stage 16. Both stages of the displacer 12 are filled with regenerative heat-exchange media in the form, for example, of tiny lead balls 18' and/or a bronze or copper screen 18". The displacer 12 reciprocates linearly within a shell 20 under the force of a motor-driven shaft 22. The shell 20 includes a first-stage cylinder 24 and a second-stage cylinder 26 conforming to and coaxial with the displacer 12 while accommodating a range of axial reciprocation of the displacer 12.
Cooling is predicated upon a reversing flow of helium gas through the shell 20 and expansion of the gas. Compressed helium gas is supplied by a compressor through a supply line 28 connected via an inlet valve 30 to the warm end 32 of the first-stage cylinder 24. With the displacer 12 at a cold end 34 of the shell 20 (remote from the inlet 35 of the supply line 28), the inlet valve 30 is opened, allowing the shell 20 to fill with compressed gas. As the compressed helium flows through the shell 20, the displacer 12 is drawn from the cold end 34 to the warm end 32 of the shell 20, forcing helium gas through passages 64, 64', 64", and 64'" of the displacer 12. The helium gas flows through the passages between the regenerative media 18', 18" filling the displacer 12, and the helium gas transfers heat to the regenerative media 18', 18", which have been precooled in previous refrigeration cycles.
When the shell 20 is filled with compressed helium and the displacer 12 is fully withdrawn to the warm end 32 of the shell 20, the inlet valve 30 is closed and the outlet valve 36 leading to a return line 38 connected to the inlet of the compressor is opened. The compressed helium gas thereby flows back through the displacer 12 and out of the shell 20, expanding into the return line 38. The helium cools with expansion, and heat is extracted from heat sinks 40, 42 (e.g., cryopanels in cryopumps) with which the refrigerator is in thermal contact. As the cooled helium flows through the displacer 12, heat is also transferred from the regenerative media (e.g., a bronze or copper screen 18" in the first stage 14 and lead balls 18' in the second stage 16) to the helium gas.
After the pressure has equilibrated between the shell 20 and the return line 38, the outlet valve 36 is closed. With the displacer 12 at the cold end 34 of the shell 20, the inlet valve 30 is reopened and the cycle is repeated.
One application for cryogenic refrigerators is in cryogenic vacuum pumps (cryopumps). Currently available cryopumps generally follow a common design. A low-temperature array, cooled to 4 to 25K (most commonly to 10 to 20K), serves as the second-stage heat sink 42 and the primary pumping surface. This array is surrounded by a higher-temperature radiation shield, usually operated in the temperature range of 40 to 130K. The radiation shield serves as the first-stage heat sink 40 to the refrigerator, and it protects the low-temperature array from radiated heat. The radiation shield generally includes a housing that is closed except at an opening where a frontal array is positioned between the primary pumping surface and a work chamber to be evacuated.
During operation, high-boiling-point gases such as water vapor are condensed on the frontal array. Lower-boiling-point gases pass through that array and into the volume within the radiation shield and condense on the low-temperature array. A surface coated with an adsorbent, such as charcoal or a molecular sieve, operating at or below the temperature of the colder array may also be provided in this volume to remove the very-low-boiling-point gases such as hydrogen. With the gases thus condensed or adsorbed on the pumping surfaces, a vacuum is created in the work chamber. Such a cryogenic refrigerator is described in U.S. Pat. No. 5,775,109, which is hereby incorporated by reference in its entirety.
Plural cryopumps, all fed by a common compressor supplying compressed helium to a common flow circuit, are often incorporated into a cluster tool for processing semiconductor wafers. Within a cluster tool, the vacuum pumps create the vacuums that are needed to perform sensitive processing steps, such as chemical vapor deposition. An embodiment of a representative cluster tool is likewise described in U.S. Pat. No. 5,775,109.